Exhaust particle removing system for an internal combustion engine

ABSTRACT

A filter disposed in an engine exhaust passage traps particles suspended in exhaust gas. A burner serves to burn off the particles deposited on the filter. The pressures in the exhaust passage at points upstream and downstream of the filter are sensed. A determination is made as to whether or not the sensed upstream pressure is lower than a preset level. When the sensed upstream pressure is not lower than the preset level, the degree of clogging of the filter is deduced on the basis of the sensed upstream and downstream pressures. When the sensed upstream pressure is lower than the preset level, the degree of clogging of the filter is deduced on the basis of a time elapsed since the moment at which the sensed upstream pressure dropped below the preset level and on the basis of the sensed upstream and downstream pressures obtained immediately prior to that moment.

BACKGROUND OF THE INVENTION

This invention relates to a system for removing particles from exhaustproduced by internal combustion engines, such as diesel engines.

Exhaust produced by diesel engines has a relatively high content ofpolluting particles composed of carbon, unburned fuel, and partiallyburned fuel. Filters disposed in engine exhaust systems areconventionally used to remove the particles from the exhaust. In thiscase, burners positioned in the exhaust systems upstream of the filtersare often employed to burn off the particles deposited on the filters inorder to unclog and rejuvenate the filters.

Japanese patent publication No. 56-115809 discloses such a particleremoving system. In this system, the pressure across the filter ismonitored as an indication of the degree of clogging of the filter. Whenthis pressure rises to a preset level, the burner is activated to unclogthe filter. The pressure across the filter, however, inaccuratelyrepresents the degree of clogging of the filter for the followingreason: this pressure depends on not only the degree of clogging of thefilter but also the rate of exhaust flow. This inaccuracy in therecognized degree of clogging of the filter could allow clogging to anunacceptable level or wasteful overuse of the burner.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an exhaust particleremoving system for an internal combustion engine which accuratelydetermines the degree of clogging of a filter and thereby reliably holdsthis degree to within an acceptable range.

In accordance with this invention, an exhaust particle removing systemincludes a filter and a burner. The filter is disposed in an engineexhaust passage to trap particles suspended in exhaust gas. The burnerserves to burn off the particles deposited on the filter. The pressuresin the exhaust passage at points upstream and downstream of the filterare sensed. A determination is made as to whether or not the sensedupstream pressure is lower than a preset level. When the sensed upstreampressure is not lower than the preset level, the degree of clogging ofthe filter is deduced on the basis of the sensed upstream and downstreampressures. When the sensed upstream pressure is lower than the presetlevel, the degree of clogging of the filter is deduced on the basis of atime elapsed since the moment at which the sensed upstream pressuredropped below the preset level and on the basis of the sensed upstreamand downstream pressures obtained immediately prior to that moment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an exhaust particle removing system according toa first embodiment of this invention.

FIG. 2 is a cross-sectional view of the burner of FIG. 1.

FIG. 3 is a graph of the relationship between the upstream pressure andthe pressure difference.

FIG. 4 is a flowchart of a program for controlling the operation of thecontrol unit of FIG. 1.

FIG. 5 is a detailed flowchart of the pressure sampling step of FIG. 4.

FIG. 6 is a diagram of the waveform of the pressure signal appearing inthe system of FIG. 1.

FIG. 7 is a detailed flowchart of the burner activation step of FIG. 4.

FIG. 8 is a detailed flowchart of the temperature control step of FIG.7.

FIG. 9 is a detailed flowchart of the burner deactivation step of FIG.4.

DESCRIPTION OF THE FIRST PREFERRED EMBODIMENT

With reference to FIG. 1, an exhaust passage 21 extends from a dieselengine (not shown) to conduct exhaust gas from the engine. A casing 22encloses a chamber. The casing 22 has an inlet 22a and an outlet 22b incommunication with each other via the chamber. This casing 22 isdisposed with respect to the exhaust passage 21 in such a way that thechamber in the casing 22 constitutes a part of the exhaust passage 21and that exhaust gas enters the chamber via the inlet 22a and exits fromthe chamber via the outlet 22b.

A filter 23 of a honeycomb structure is disposed within the downstreamhalf of the casing 22 to trap particles suspended in the exhaust gas. Abuffer 24 is sandwiched between the filter 23 and the walls of thecasing 22 so that the filter 23 is gently supported within the casing22. This filter 23 has a plurality of parallel holes extending betweenthe upstream and downstream ends of the filter 23. These holes are oftwo types. The holes of the first type have open upstream ends andclosed downstream ends. The holes of the second type have closedupstream ends and open downstream ends. The first holes adjoin thesecond holes via porous walls of the filter 23 so that the exhaust gasfirst enters the first holes and then passes through the porous wallsinto the second holes before exiting via the second holes. As theexhaust gas passes through the porous walls, particles suspended inthese gases are trapped by the walls.

A burner 25 disposed within the upstream half of the casing 22 serves toburn off the particles trapped by the filter 23. As shown in FIGS. 1 and2, the burner 25 includes a cylindrical combustion liner 26, a mixturevaporizing tube 27, a mixture injection tube 28, and a glow plug 29. Thecombustion liner 26 defines a combustion chamber in a region immediatelyupstream of the filter 23. The combustion liner 26 has a plurality ofapertures 26a which admit exhaust gas into the combustion chamber. Afterpassing through the combustion chamber, the exhaust gas enters thefilter 23. The mixture vaporizing tube 27 is disposed within thecombustion liner 26. The mixture injection tube 28 extends through thewalls of the liner 26 into the mixture vaporizing tube 27. The mixtureinjection tube 28 serves to discharge a mixture of air and fuel into themixture vaporizing tube 27. The discharged mixture reverses its flow andthen flows out of the mixture vaporizing tube 27 into the combustionchamber via an opening or openings 27a through the walls of the tube 27.The glow plug 29 projects into a section of the combustion chamber nearthe opening 27a to ignite the mixture entering the combustion chamber.After igniting, the mixture burns in the combustion chamber and therebythe particles trapped by the filter 23 are burned off of the filter 23.Furthermore, the combustion of the mixture heats the mixture vaporizingtube 27, thereby speeding evaporation of the fuel in the tube 27 andthus facilitating ignition of the subsequently supplied mixture.

As shown in FIG. 2, the mixture vaporizing tube 27 and a member 27bfixed to the tube 27 define an ignition chamber 27c in communicationwith both of the opening 27a and the combustion chamber. The glow plug29 includes an elongated cylinder 29a and a sleeve cover 29b. Thecylinder 29a extends into the ignition chamber 27c and is exposed to thechamber 27c. This exposed section of the cylinder 29a contains a heatingelement. The sleeve cover 29b surrounds a segment of the cylinder 29aextending in the combustion chamber and the space between the combustionliner 26 and the casing 22 in order to protect this segment of thecylinder 29a from a blaze. The cover 29b is preferably made of a heatresisting alloy.

Returning to FIG. 1, an electromagnetic-type fuel supply control valve30 has an outlet connected to the mixture injection tube 28 via a fuelfeed pipe 31. Fuel is driven from a fuel tank 32 to an inlet of the fuelvalve 30 by an electrically-powered fuel pump 33. This fuel may consistof the same petroleum fuel used to run the engine. When the fuel valve30 is open, fuel is admitted into the mixture injection tube 28 via themixture feed pipe 31 provided that the fuel pump 33 is activated. Whenthe fuel valve 30 is closed, fuel supply is interrupted. Electricalenergization and de-energization of the fuel valve 30 causes the fuelvalve 30 to be opened and closed respectively.

An engine-driven air pump 34 has an inlet 34a and an outlet 34b. Theinlet 34a leads to the open air via an air cleaner (not shown). The airpump 34 draws air via the inlet 34a and discharges it via the outlet34b. A pressure-operated three-way valve 35 has a control chamberpartially defined by a spring-loaded diaphragm, a first port 35a, asecond port 35b, and a third port 35c. The first port 35a is connectedto the air pump outlet 34b. The second port 35b leads to the open airvia the air cleaner. The third port 35c is connected to the mixtureinjection tube 28 via an air feed pipe 36. When the control chamber issupplied with atmospheric pressure, the first port 35a is connected tothe second port 35b and is disconnected from the third port 35c so thatair driven by the air pump 34 is relieved via the valve 35 and thus themixture injection tube 28 does not receive any air. When the controlchamber is supplied with a preset vacuum, the first port 35a isconnected to the third port 35c and is disconnected from the second port35b so that air driven by the air pump 34 enters the mixture injectiontube 28 via the valve 35 and the air feed pipe 36.

Adjustment of the pressure in the control chamber of the valve 35 isrealized via an electromagnetic three-way valve 37 having a first port37a, a second port 37b, and a third port 37c. The first port 37a isconnected to the control chamber of the valve 35. The second port 37b isconnected to a vacuum source, such as an engine-driven vacuum pump. Thethird port 37c leads to the open air via the air cleaner. When theelectromagnetic valve 37 is electrically de-energized, the first port37a is connected to the third port 37c and is disconnected from thesecond port 37b so that atmospheric pressure is supplied to the controlchamber of the valve 35, thereby interrupting the air supply to themixture injection tube 28. When the electromagnetic valve 37 iselectrically energized, the first port 37a is connected to the secondport 37b and is disconnected from the third port 37c so that the presetvacuum is supplied to the control chamber of the valve 35, therebyenabling the air supply to the mixture injection tube 28.

A control unit 50 includes a switch section 51 having switches 51a, 51b,51c, 51d, and 51e. Each of these switches 51a, 51b, 51c, 51d, and 51econsists mainly of a switching power transistor. The fuel pump 33 iselectrically connected across a battery 52 via the switch 51a and anengine key switch 70. Provided that the key switch 70 is closed, thefuel pump 33 is electrically energized and de-energized when the switch51a is closed and opened respectively. The fuel valve 30 is electricallyconnected across the battery 52 via the switch 51b and the key switch70. Provided that the key switch 70 is closed, the fuel valve 30 iselectrically energized and de-energized when the switch 51b is closedand opened respectively. The electromagnetic valve 37 is electricallyconnected across the battery 52 via the switch 51c and the key switch70. Provided that the key switch 70 is closed, the electromagnetic valve37 is electrically energized and de-energized when the switch 51c isclosed and opened respectively. A relay 53 has a control winding 53a anda switch 53b. Electrical energization and de-energization of the controlwinding 53a causes the switch 53b to be closed and opened respectively.The glow plug 29 is electrically connected across the battery 52 via therelay switch 53b and the key switch 70. Provided that the key switch 70is closed, the glow plug 29 is electrically energized and de-energizedwhen the relay switch 53b is closed and opened respectively. The relaywinding 53a is electrically connected across the battery 52 via theswitch 51d and the key switch 70. Provided that the key switch 70 isclosed, the relay winding 53a is electrically energized and de-energizedwhen the switch 51d is closed and opened respectively. Accordingly,provided that the key switch 70 is closed, closing and opening of theswitch 51d causes the glow plug 29 to be activated and de-activatedrespectively. The switches 51a, 51b, 51c, and 51d have control terminalsto which electrical signals S1, S2, S3, and S4 are applied respectivelyto control the switches 51a, 51b, 51c, and 51d. It should be noted thatthe engine key switch 70 remains closed while the engine is running.

Under conditions where the key switch 70 is closed, when all of theswitches 51a, 51b, 51c, and 51d are closed, the fuel pump 33, the fuelvalve 30, the electromagnetic valve 37, and the glow plug 29 are allelectrically energized. As a result, fuel is driven by the fuel pump 33into the mixture injection tube 28 via the fuel valve 30 and air isadmitted from the air pump 34 into the mixture injection tube 28 via theair valve 35. A mixture of air and fuel thus results, which isdischarged into the combustion chamber of the burner 25. The dischargedmixture is ignited by the glow plug 29. Thus, in this case, the burner25 is activated.

In more detail, the fuel valve 30 is energized intermittently at aconstant frequency during operation of the burner 25. The rate of fuelsupply to the burner 25 is determined by the duty cycle of currentpulses flowing through the fuel valve 30.

When all of the switches 51a, 51b, 51c, and 51d are opened, the fuelpump 33, the fuel valve 30, the electromagnetic valve 37, and the glowplug 29 are all electrically de-energized. As a result, the fuel valve30 interrupts admission of fuel into the mixture injection tube 28 andthe air valve 35 interrupts admission of air into the mixture injectiontube 28, so that no air-fuel mixture is supplied to the burner 25. Thus,in this case, the burner 25 is de-activated.

An electromagnetic three-way valve 80 has three ports 80a, 80b, and 80c.When this valve 80 is electrically de-energized, the first port 80a isconnected to the second port 80b and is disconnected from the third port80c. When the valve 80 is electrically energized, the first port 80a isconnected to the third port 80c and is disconnected from the second port80b.

A first pressure introduction passage 38 connects the interior of thecasing 22 to the second port 80b of the three-way valve 80. The junctionof this passage 38 and the interior of the casing 22 is located at apoint immediately upstream of the filter 23 so that the second port 80bis supplied with the pressure in the casing 22 at this point. Thispressure will be referred to as the upstream pressure P1 hereafter. Asecond pressure introduction passage 39 connects the interior of thecasing 22 to the third port 80c of the three-way valve 80. The junctionof this passage 39 and the interior of the casing 22 is located at apoint immediately downstream of the filter 23 so that the third port 80cis supplied with the pressure in the casing 22 at this point. Thispressure will be referred to as the downstream pressure P2 hereafter.

A pressure sensor 82 has a sensing port 82a and a reference port 82b.This pressure sensor 82 detects the pressure difference between itsports 82a and 82b. The sensing port 82a is connected to the first port80a of the three-way valve 80. A diaphragm 84 is disposed in theconnection between the sensing port 82a and the first valve port 80a toprevent the transmission of heat and moisture but allow the transmissionof pressure between them. In this way, the sensing port 82a is suppliedwith the pressure developed in the first valve port 80a. The referenceport 82b leads to the open air via the air cleaner to be exposed toatmospheric pressure. The pressure sensor 82 is supplied with a constantvoltage Vcc from a voltage regulator 60 described in more detailhereafter. The pressure sensor 82 outputs a voltage signal VP whichrepresents the pressure difference between its ports 82a and 82b. Amultiplexer 57 in the control unit 50 is electrically connected to thepressure sensor 82 to receive the pressure signal VP.

The pressure sensor 82 may have a piezoelectric element and a gaugeresistor provided on a silicon diaphragm whose opposing surfaces aresubjected to the pressures in its ports 82a and 82b respectively. Inthis case, the effective resistance of the gauge resistor varies as afunction of the difference in pressure between the ports 82a and 82b.The voltage regulator 60 disposed in the control unit 50 and connectedto the battery 52 derives a constant voltage Vcc from the voltage VBacross the battery 52. This constant voltage Vcc is applied across aseries combination of a reference fixed resistor and the gauge resistor,so that the voltage across the gauge resistor or the voltage across thefixed resistor varies as a function of the resistance of the gaugeresistor. Since the resistance of the gauge resistor depends on thepressure difference, the voltage across the gauge resistor or thevoltage across the fixed resistor represents that pressure difference.This voltage is outputted by the sensor 41 as a pressure signal VP.

The three-way valve 80 is electrically connected across the battery 52via the switch 51e in the control unit 50 and via the key switch 70.Provided that the key switch 70 is closed, the three-way valve 80 iselectrically energized and de-energized when the switch 51e is closedand opened respectively. A control signal S5 applied to a controlterminal of the switch 51e selectably determines the operating positionof the switch 51e.

Under conditions where the key switch 70 is closed, when the switch 51eis opened, the three-way valve 80 is de-energized so that the firstvalve port 80a is connected to the second valve port 80b and isdisconnected from the third valve port 80c. As a result, the upstreampressure P1 is supplied to the sensor sensing port 82a so that thepressure sensor 82 measures this upstream pressure P1 with respect toatmospheric pressure. Under the same conditions, when the switch 51e isclosed, the three-way valve 80 is energized so that the first valve port80a is connected to the third valve port 80c and is disconnected fromthe second valve port 80b. As a result, the downstream pressure P2 issupplied to the sensor sensing port 82a so that the pressure sensor 82measures this downstream pressure P2 with respect to atmosphericpressure.

A temperature sensor 44 is disposed within the casing 22 at a pointdirectly upstream of the center of the end face of the filter 23. Thetemperature sensor 44 outputs a voltage VT representing the temperatureat that point. This temperature sensor 44 may consist basically of athermocouple.

An engine speed sensor 45 monitors the rotational speed of the engine.This speed sensor 45 includes a crank angle sensor generating pulsescorresponding to evenly spaced angular positions of the enginecrankshaft. The frequency of these pulses is thus proportional to theengine speed. The speed sensor 45 outputs the resulting pulse signalRev, the frequency of which indicates the engine speed. Afrequency-to-voltage (F/V) converter 56 in the control unit 50 isconnected to the engine speed sensor 45 to receive the pulse signal Rev.This circuit 56 converts the pulse signal Rev to a voltage VR whichvaries as a function of the engine speed.

An engine load sensor 46 monitors the load on the engine. This loadsensor 46 includes a potentiometer 46a, the adjustment shaft of which islinked to the control lever 47a of a fuel injection pump 47. The angularposition of the control lever 47a determines the rate of fuel injectioninto the engine. This control lever 47a is connected to an accelerator(not shown) so that the angular position of the control lever 47areflects the power output required of the engine, that is, the load onthe engine. The constant voltage Vcc outputted by the voltage regulator60 is applied across the resistor of the potentiometer 46a. As a result,the potentiometer 46a outputs a voltage VL which varies as a function ofthe engine load.

The control unit 50 includes a digital central processing unit (CPU) 54,a read-only memory (ROM) 55, a multiplexer (MPX) 57, ananalog-to-digital (A/D) converter 58, and a peripheral input/output(PIO) circuit 59.

The multiplexer 57 is connected to the sensors 82, 44 and 46 and the F/Vconverter 56 to receive the pressure signal VP, the temperature signalVT, the engine load signal VL, and the engine speed signal VR. Themultiplexer 57 selects one of these signals in accordance with a channelselection signal CH issued by the PIO circuit 59 and passes it to theA/D converter 58. This signal CH has four different states correspondingto the four different selections. After receiving a start signal STARTfrom the PIO circuit 59, the A/D converter 58 commences converting theselected signal to a corresponding digital signal DATA. Upon completionof the conversion, the A/D converter 58 outputs an end-of-conversionsignal EOC to the PIO circuit 59 and then the digital signal DATA istransmitted to the PIO circuit 59.

The PIO circuit 59 also outputs the control signals S1, S2, S3, S4, andS5 to the switches 51a, 51b, 51c, 51d, and 51e of the section 51 viaconnections between the circuit 59 and the switches 51a, 51b, 51c, 51d,and 51e.

The CPU 54 is connected to the PIO circuit 59 and the ROM 55 holding aprogram and fixed data. The CPU 54 has an internal random-access memory(RAM). The CPU 54, the ROM 55, and the PIO circuit 59 are connected tothe voltage regulator 60 so as to be powered by the constant voltageVcc. It should be noted that the other circuits 51, 56, 57, and 58 arealso powered by this constant voltage Vcc.

Since exhaust gas generally exhibits laminar flow in the filter 23, thepressure P1 in the exhaust passage 21 at a point upstream of the filter23, the pressure P2 in the exhaust passage 21 at a point downstream ofthe filter 23, and the pressure difference ΔP across the fitler 23 areall approximately proportional to the rate of exhaust gas flow throughthe filter 23 provided that the resistance of the filter 23, that is,the degree of clogging of the filter 23 is constant. The pressuredifference ΔP equals the upstream pressure P1 minus the downstreampressure P2. At a fixed degree of clogging of the filter 23, the ratiobetween the pressures P1 and P2 as well as the ratio between theupstream pressure P1 and the pressure difference ΔP thus remain atapproximately constant levels independent of the rate of exhaust gasflow.

As the degree of clogging of the filter 23 increases, the ratio ΔP/P1also increases. In addition, the ratio ΔP/P1 is independent of the rateof exhaust gas flow. Accordingly, this ratio ΔP/P1 represents the degreeof clogging of the filter 23.

FIG. 3 shows the typical relationship between the upstream pressure P1and the pressure difference ΔP. In this graph, the lower line Arepresents a minimum level of clogging of the filter 23, while the upperline B represents a reference level separating acceptable andunacceptable ranges of clogging of the filter 23. When the ratio of theupstream pressure P1 to the difference pressure ΔP reaches thisreference level, the burner 25 should be activated to unclog the filter23.

At low engine speeds and small engine loads near values realized underengine idling conditions, the upstream pressure P1 is so low that theratio ΔP/P1 critically or sensitively depends on fluctuations in exhaustpressures and that the accuracy of calculation of the ratio ΔP/P1 isconsiderably affected by the accuracy of pressure sensing. As a result,errors in deducing the degree of clogging of the filter 23 directly fromthe ratio ΔP/P1 may be significant in this engine speed and load range.This invention solves this problem as will be apparent from thefollowing description.

The control unit 50 operates in accordance with a program stored in theROM 55. FIG. 4 is a flowchart of this program.

In the initial step 101 of this program, the current value of the enginespeed derived from the engine speed signal VR is stored in the RAM ofthe CPU 54. In this flowchart, the variable N represents this enginespeed value. After the step 101, the program advances to a step 102.

In the step 102, the CPU 54 determines whether or not the engine hasstarted, specifically whether or not the engine speed value N exceeds apreset level Nref1 preferably chosen to be 500 rpm. If the engine hasnot yet started, that is, if the engine speed value N does not exceedthe preset level Nerf1, the program returns from the step 102 to theinitial step 101 via a step 119 in which the burner 25 is deactivated.If the engine has started, that is, if the engine speed value N exceedsthe preset level Nref1, the program advances from the step 102 to a step103.

in the step 103, the current value of the engine load derived from theengine load signal VL is stored in the RAM of the CPU 54. In thisflowchart, the variable Q represents this engine load value. After thestep 103, the program advances to a step 104.

In the step 104, the CPU 54 determines whether or not the burner 25 isactive. If the burner 25 is active, the program advances via a step 117to a step 118 in which activation of the burner 25 is maintained. If theburner 25 is inactive, the program advances to a step 105.

In the step 105, the current values of the upstream and downstreampressures P1 and P2 are sampled and stored as described in more detailhereafter. In this flowchart, the variables VP1 and VP2 represent theupstream and downstream pressure values respectively. As will be madeclear later, this step 105 branches either back to the initial step 101or onward to a step 106.

In the step 106, the CPU 54 determines whether or not the upstreampressure value VP1 is equal to or greater than a preset reference valueVP1ref. This reference value VP1ref is chosen so as to represent thelower limit of an acceptable range of the upstream pressure valueswithin which derivation of the degree of clogging of the filter 23directly from the ratio ΔP/P1 is accurate and reliable. If the upstreampressure value VP1 is equal to or greater than the reference valueVP1ref, that is, if the degree of clogging of the filter 23 can bederived directly from the ratio ΔP/P1 with acceptable accuracy andreliability, the program advances to a step 107. If the upstreampressure value VP1 is smaller than the reference value VP1ref, that is,if the above derivation can be neither accurate nor reliable, theprogram advances to a step 111.

In the step 107, the CPU 54 determines the ratio of the upstreampressure value VP1 to the pressure difference value VΔP, whereVΔP=(VP1-VP2) and VP2 is the downstream pressure value. In thisflowchart, this ratio is represented by the variable K0. Specifically,"K0=(VP1-VP2)/VP1" is executed. It should be noted that(VP1-VP2)/VP1=1-(VP2/VP1).

In a step 108 following the step 107, the CPU 54 determines whether ornot this is the first cycle of execution of the program since the lastunclogging of the filter 23 was completed. Specifically, a determinationis made as to whether or not the value of the variable K1 held in theRAM of the CPU 54 is zero. In general, this variable K1 represents thepreceding value of the variable K as will be made clear with referenceto a step 116. In any case, the variable K1 will be zero during thefirst cycle of execution of the program following the completion of anunclogging operation. If the variable K1 is zero, that is, if this isthe first cycle of execution, the program proceeds to a step 110. If thevariable K1 is not zero, that is, if this is not the first cycle ofexecution, the program proceeds to a step 109.

It should be noted that since the CPU 54 is continuously powered by theconstant voltage Vcc from the voltage regulator 60 independent of theposition of the engine key switch 70 as understood from FIG. 1, the RAMof the CPU 54 holds the data even while the engine remains at rest.

In the step 110, the ratio value K0 is stored in a location in the RAMof the CPU 54 which is designated by the variable K in this flowchart.In other words, "K=K0" is executed. After the step 110, the programadvances to a step 115.

In the step 109, "K=(K0+7K1)/8" is executed. In this equation, K1 is thevariable representing the preceding value of the variable K as will bemade clear with reference to a step 116 described hereafter.Accordingly, this step 109 calculates the weighted mean value of thecurrent ratio value K0 and the preceding value of the variable K havingcomponents of the previous ratio values K0. This current mean value ofthe variable K is used as a final indication of the degree of cloggingof the filter 23. The weighted mean prevents erroneous determination ofthe degree of clogging of the filter 23 resulting from questionableabrupt changes in the ratio value K0. The weighting factor of thisaveraging operation may differ from that used in this embodiment. Afterthe step 109, the program advances to the step 115.

In the step 115, the CPU 54 determines whether or not the value of thevariable K is equal to or greater than a preset value Kmax. Thisreference value Kmax is chosen to correspond to the boundary betweenacceptable and unacceptable ranges of clogging of the filter 23. If thevalue K is smaller than the reference value Kmax, that is, if the degreeof clogging of the filter 23 is acceptable, the program proceeds to thestep 116. If the value K is equal to or greater than the reference valueKmax, that is, if the degree of clogging of the filter 23 isunacceptable, the program proceeds to a step 117.

In the step 116, the value of the variable K is stored in a location inthe RAM of the CPU 54 which is designated by the variable K1 in thisflowchart. After the step 116, the program returns to the initial step101.

In the step 117, the value of the variable K1 is set to zero.Specifically, "K1=0" is executed. After the step 117, the programproceeds to a step 118.

In the step 118, the control unit 50 activates the burner 25 to unclogthe filter 23. As a result of cooperation of the steps 115 and 118, whenthe degree of clogging of the filter 23 reaches an unacceptable level,the burner 25 is automatically activated to unclog the filter 23. Thisstep 118 branches to either the initial step 101 or the burnerdeactivation step 119. After the step 119, the program returns to theinitial step 101.

In the step 111, the CPU 54 determines whether or not the time elapsedsince the moment at which the upstream pressure value VP1 drops belowthe reference value VP1ref exceeds a preset interval Tref1. As will bemade clear hereinafter, this interval Tref1 determines a timing at whichthe value of the variable K is to be corrected. If this elapsed timedoes not exceed the preset interval Tref1, the program returns to theinitial step 101. If this elapsed time exceeds the preset intervalTref1, the program advances to a step 112. It should be noted that asshown in FIG. 1, the CPU 54 includes a combination of a fixed frequencyclock 54a and counters 54b for measuring this elapsed time and othertimes described hereafter. As a result of cooperation of the steps 106and 111, the program advances to the step 112 provided that the upstreampressure value VP1 remains below the preset value VP1ref for more thanthe preset interval Tref1.

In the step 112, the CPU 54 determines whether or not this is the firstcycle of execution of the program since the last unclogging of thefilter 23 was completed. Specifically, a determination is made as towhether or not the value of the variable K1 held in the RAM of the CPU54 is zero. If the value of the variable K1 is not zero, that is, ifthis is not first execution cycle of the program, the program proceedsto a step 113. If the value of the variable K1 is zero, that is, if thisis the first execution cycle of the program, the program advances to astep 114.

In the step 113, the CPU deduces the increase in the value of thevariable K which could be expected over a preset length of time equal tothe preset interval Tref1 on the basis of the engine speed value N andthe engine load value Q sampled in the steps 101 and 103. In thisflowchart, the variable ΔK represents this increase value. The increasevalue ΔK is chosen to correspond to the increase in the degree ofclogging of the filter 23 over the interval Tref1. It should be notedthat the rate of increase in the degree of clogging of the filter 23generally depends on the engine speed and the engine load. Specifically,the ROM 55 holds a table in which a set of values of the increase valueΔK are plotted as a function of the engine speed and the engine load. Byreferring to this table, the CPU 54 determines the increase value ΔK.After determining the increase value ΔK, the CPU 54 executes "K=K1+ΔK".In other words, the CPU 54 calculates the sum of the values of thevariables K1 and ΔK and then writes the result in the variable K. Inthis way, the value of the variable K, serving as a final indication ofthe degree of clogging of the filter 23, is corrected in cases where theupstream pressure value VP1 drops below the reference value VP1ref andthen remains lower than the value VP1ref for longer than the presetinterval Tref1. From this step 113, the program advances to the step115.

The increase value ΔK may be equal to a predetermined constantindependent of the current engine speed and load. In this case, theincrease value ΔK is chosen on the basis of typical values of the enginespeed and load occurring under conditions where the upstream pressurevalue VP1 is lower than the reference value VP1ref.

In the step 114, the CPU 54 sets the variable K to a preset value Kmin.This preset value Kmin is chosen to correspond to the value(VP1=VP2)/VP1 prevailing when the degree of clogging of the filter 23 iszero or minimal. After the step 114, the program advances to the step115.

As a result of return of the program to the initial step 101, thisprogram is reiterated so that the engine speed value N, the engine loadvalue Q, the upstream pressure value VP1, the downstream pressure valueVP2, the ratio value K0, and the value of the variable K are allcontinuously updated.

In a re-initialization step (not shown) between the steps 111 and 112,the CPU 54 resets an internal counter for defining a variablerepresenting the time elapsed since the moment at which the upstreampressure value VP1 drops below the preset value VP1ref. Therefore, aslong as the upstream pressure value VP1 continues to be lower than thepreset value VP1ref, one of the step 113 or the step 114 will beexecuted at regular intervals equal to the preset interval Tref1 used inthe step 111. It should be noted that a similar re-initialization stepresides between the steps 106 and 107.

FIG. 5 is a detailed flowchart of the pressure sampling step 105. In astep 121 following the step 104 (see FIG. 4), the CPU 54 determineswhether or not the engine is running normally, specifically whether ornot the engine speed value N exceeds a preset level Nref2 preferablychosen to be 500 rpm. If the engine has stopped, that is, if the enginespeed value N does not exceed the preset level Nref2, the programreturns to the initial step 101 (see FIG. 4) via a step 130 in which thepressure sampling operation is suspended and reinitialized. If theengine is running normally, that is, if the engine speed value N exceedsthe preset level Nref2, the program advances to a step 122.

In the step 122, the CPU 54 determines whether or not an upstreampressure value VP1 derived from the pressure signal VP has been storedin the RAM of the CPU 54 since the last execution of an intializationstep 129 described hereafter. If no upstream pressure value VP1 has yetbeen stored, the program proceeds to a step 123. If an upstream pressurevalue VP1 has already been stored, the program advances to a step 126.

In the step 123, the electromagnetic valve 80 (see FIG. 1) isde-energized or is held in the de-energized state. Specifically, thecontrol signal S5 designed to open the switch 51e (see FIG. 1) isoutputted to the control terminal of the switch 51e. As describedpreviously, opening the switch 51e causes de-energization of theelectromagnetic valve 80, thereby allowing the pressure sensor 82 (seeFIG. 1) to measure the upstream pressure P1 relative to atmosphericpressure.

In a step 124 subsequent to the step 123, the CPU 54 determines whetheror not the time elapsed since the moment of de-energization of theelectromagnetic valve 42 exceeds a preset interval Tref2. If thiselapsed time does not exceed the preset interval Tref2, the programreturns to the initial step 101. As a result, the electromagnetic valve80 continues to be de-energized until the elapsed time reaches thepreset interval Tref2. If the elapsed time exceeds the preset intervalTref2, the program advances to a step 125.

In the step 125, the current upstream pressure value VP1 derived fromthe pressure signal VP is stored in the RAM of the CPU 54. After thestep 125, the program proceeds to the step 126.

In the step 126 following one of the steps 122 and 125, theelectromagnetic valve 80 is energized. Specifically, the control signalS5 designed to close the switch 51e is outputted to the control terminalof the switch 51e. As described previously, closing the switch 51eresults in energization of the electromagnetic valve 80, therebyallowing the pressure sensor 82 to measure the downstream pressure P2relative to atmospheric pressure.

In a step 127 subsequent to the step 126, the CPU 54 determines whetheror not the time elapsed since the moment of energization of theelectromagnetic valve 80 exceeds a preset interval Tref3. If thiselapsed time does not exceed the preset interval Tref3, the programreturns to the initial step 101. As a result, the electromagnetic valve80 continues to be energized until the elapsed time reaches the presetinterval Tref3. If the elapsed time exceeds the preset interval Tref3,the program advances to a step 128.

In the step 128, the current downstream pressure value VP2 derived fromthe pressure signal VP is stored in the RAM of the CPU 54. After thestep 128, the program advances to the step 106 (see FIG. 4) via the step129 in which the pressure derivation operation is initialized,specifically in which the variables relating to the pressure derivationoperation are cleared and initialized.

The upstream and downstream pressure values VP1 and VP2 are sampled ineach cycle of execution of the main flow of this pressure derivationstep 105, so that these values VP1 and VP2 are periodically sampled andupdated.

FIG. 6 shows a typical waveform of the pressure signal VP. Theelectromagnetic valve 80 switches from the de-energized state to theenergized state at moments denoted by the letter C. The electromagneticvalve 80 switches from the energized state to the de-energized state atmoments denoted by the letter D. During the time intervals Tref2determined in the step 124 (see FIG. 5), the electromagnetic valve 80remains de-energized. During the time intervals Tref3 determined in thestep 127 (see FIG. 5), the electromagnetic valve 80 remains energized.After a certain time lag following each change in the state of theelectromagnetic valve 80, the pressure signal VP reflects the truepressure value. The time intervals Tref2 and Tref3 are chosen to belonger than this time lag. The time interval Tref2 is preferably longerthan the time interval Tref3 to decrease the frequency of changes in thestates of the electromagnetic valve 80 in order to maximize the servicelife of the valve 80. The shorter time interval Tref3 is preferably 0.2seconds. The upstream pressure VP1 is sampled immediately prior to themoments C at which the electromagnetic valve 80 is energized. Thedownstream pressure VP2 is sampled immediately prior to the moments D atwhich the electromagnetic valve 80 is de-energized. The interval betweenthe sampling of the paired values VP1 and VP2 in that order isessentially equal to the preset interval Tref3. Since this intervalTref3 is short, the paired values VP1 amd VP2 reflect pressuresprevailing at essentially the same moment. This arrangement ensures theaccuracy of these values VP1 and VP2 even under rapidly changingoperating conditions of the engine.

FIG. 7 is a detailed flowchart of the burner activation step 118. In thestep 141 following the step 117 (see FIG. 4), the CPU 54 determineswhether or not the engine is running normally, specifically whether ornot the engine speed value N exceeds a preset level Nref3 preferablychosen to be 500 rpm. If the engine has stopped, that is, if the enginespeed value N does not exceed the preset level Nref3, the programadvances to the burner deactivation step 119 (see FIG. 4). If the engineis running normally, that is, if the engine speed value N exceeds thepreset level Nref3, the program advances to a step 142.

In the step 142, the CPU 54 determines whether or not ignition of theair-fuel mixture has been achieved on the basis of the currenttemperature value derived from the temperature signal VT. Specifically,a check is made for whether or not a preset increase in the temperaturevalue occurs within a predetermined interval after commencement of airand fuel supply, that is, whether or not the temperature value reaches apreset level within the predetermined interval. If ignition has beenachieved, that is, if the temperature value exceeds the preset level,the program advances to a step 150. If ignition has not been achieved,that is, if the temperature value does not exceed the preset level, theprogram advances to a step 143. Generally, ignition is not achievedimmediately, so that the program will repeatedly advance from the step142 to the step 143 at early stages.

In the step 143, the control unit 50 activates the glow plug 29.Specifically, the control signal S4 designed to close the switch 51d isoutputted to the control terminal of the switch 51d. As describedpreviously, closing the switch 51d results in activation of the glowplug 29. After the step 143, the program proceeds to a step 144.

In the step 144, the CPU 54 determines whether or not the elapsed timesince the commencement of activation of the glow plug 29 exceeds apreset interval Tref4. This reference interval Tref4 is chosen so thatthe sustained activation of the glow plug 29 over this period enablesthe temperature around the glow plug 29 to assume a level high enough toignite the air-fuel mixture. The preset interval Tref4 is preferably 50seconds. If elapsed time from commencement of activation of glow plug 29does not exceed the preset interval Tref4, the program returns to theinitial step 101 (see FIG. 4). As a result, activation of the glow plug29 is sustained for the preset interval provided that the engine keepsrunning. When this elapsed time exceeds the preset interval Tref4, theprogram proceeds to a step 145.

In the step 145, the CPU 54 determines whether or not a temperaturevalue derived from the temperature signal VT immediately prior toignition of the air-fuel mixture has already been stored in its internalRAM. If a temperature value has not yet been stored, the programadvances to a step 152 in which the current value of temperature derivedfrom the temperature signal VT is stored in the RAM of the CPU 54. Inthis flowchart, the variable VT0 represents this temperature value. Aswill be made clear, the temperature value VT0 is sampled immediatelyprior to ignition of the air-fuel mixture. After the step 152, theprogram advances to a step 146. If a temperature value has already beenstored, the program advances from the step 145 directly to the step 146.

In the step 146, the control unit 50 performs air supply and fuel supplyto the burner 25. Specifically, the control signal S3 designed to closethe switch 51c (see FIG. 1) is outputted to the control terminal of theswitch 51c. As described previously, closing the switch 51c results inair supply to the burner 25. The control signal S1 designed to close theswitch 51a (see FIG. 1) is outputted to the control terminal of theswitch 51a. As described previously, closing the switch 51a results inactivation of the fuel pump 33 (see FIG. 1). The control signal S2designed to close the switch 51b (see FIG. 1) intermittently at a fixedfrequency is outputted to the control terminal of the switch 51b. Asdescribed previously, this periodic closing of the switch 51b results inthe corresponding periodic opening of the fuel valve 30 (see FIG. 1).Accordingly, fuel is supplied to the burner 25. The rate of fuel supplydepends on the duty cycle of the control signal S2.

In more detail, in the fuel supply section of the step 146, the CPU 54determines a desired basic value of the pulse width of the driving pulsecurrent to the fuel valve 30 on the basis of the engine speed value Ngiven in the step 101 and the engine load value Q given in the step 103.Specifically, the ROM 55 holds a table in which a set of desired basicpulse-width values are plotted as a function of the engine speed value Nand the engine load value Q. The determination of the basic pulse-widthvalue is carried out by referring to this table. It should be noted thatthe pulse-widths of the current pulses driving the fuel valve 30correspond to the interval of time for which the fuel valve 30 remainsopen. The current pulses driving the fuel valve 30 have a fixedfrequency, preferably chosen to be 25 Hz. The desired basic pulse-widthvalues are preferably in the range of 0 to 40 milliseconds. Then, theCPU 54 determines a desired final pulse-width value of the currentpulses driving the fuel valve 30 on the basis of the desired basicpulse-width value. Specifically, the desired final pulse-width value isequal to the desired basic pulse-width value multiplied by a factor of2.0 so as to facilitate ignition. Finally, the control signal S2 in theform of a pulse train having a constant frequency equal to the fixedvalue and having a pulse-width equal to the desired final pulse-widthvalue is outputted to the control terminal of the switch 51b so that theswitch 51b is closed intermittently at the same fixed frequency and theduration of each closing of the switch 51b is maintained at a valueequal to the desired final pulse-width value. This periodic closing ofthe switch 51b actuates a correspondingly periodic opening of the fuelvalve 30. This opening of the fuel valve 30 allows fuel supply to theburner 25 at a rate determined by the desired final pulse-width value.

In a step 147 subsequent to the step 146, the current value of thetemperature is derived from the temperature signal VT. In thisflowchart, the variable VT1 represents this temperature value. Thetemperature value VT1 is sampled immediately following ignition of theair-fuel mixture. Then, the CPU 54 determines the difference between thetemperature value VT1 and the temperature value VT0 given in the step152. Specifically, "ΔVT=VT1-VT0" is executed, where ΔVT is a variablerepresenting the temperature difference. Since the variables VT0 and VT1represent the values of temperature at moments immediately preceding andfollowing ignition of the air-fuel mixture, the variable ΔVT representsthe increase in the temperature caused by ignition. Finally, the CPU 54determines whether or not the temperature difference ΔVT exceeds apreset level Href1 preferably chosen to be 100° C. If the temperaturedifference ΔVT exceeds the preset level Href1, the program advances to astep 148. If the temperature difference ΔVT does not exceed the presetlevel Href1, the program advances to a step 153.

In the step 153, the CPU 54 determines whether or not the time elapsedsince commencement of air-fuel mixture supply exceeds a preset intervalTref5 equal to the longest value necessary for the temperaturedifference ΔVT to reach the preset level Href1. If this elapsed timedoes not exceed the preset interval Tref5, the program returns to theinitial step 101 (see FIG. 4). If the elapsed time exceeds the presetinterval Tref5, the program advances to the burner deactivation step 119(see FIG. 4) and then returns to the initial step 101. The referenceinterval Tref5 is preferably 10 seconds.

In the step 148, the CPU 54 determines whether or not the temperaturevalue VT1 exceeds a preset level Href2 at which self-sustainingcombustion of the air-fuel mixture will occur. This preset level Href2is preferably 500° C. If the temperature value VT1 exceeds the presetlevel Href2, the program advances to a step 149. If the temperaturevalue VT1 does not exceed the preset level Href2, the program advancesto a step 154.

In the step 154, the CPU 54 determines whether or not the time elapsedsince ignition of the air-fuel mixture exceeds a preset interval Tref6preferably chosen to be 40 seconds. If this elapsed time does not exceedthe preset interval Tref6, the program returns to the initial step 101(see FIG. 4). If the elapsed time exceeds the preset interval Tref6, theprogram advances to the burner deactivation step 119 (see FIG. 4) andthen returns to the initial step 101. Accordingly, in the case whereself-sustaining combustion is not attained within the fixed length oftime defined by the preset interval Tref6, the burner 25 is deactivated.

In the step 149, the control unit 50 deactivates the glow plug 29.Specifically, the control signal S4 designed to open the switch 51d isoutputted to the control terminal of the switch 51d. As describedpreviously, opening the switch 51d results in deactivation of the glowplug 29. Accordingly, in the case where self-sustaining combustion isattained, the glow plug 29 is deactivated. After the step 149, theprogram proceeds to the step 150.

In the step 150 following one of the steps 142 and 149, the control unit50 performs temperature control designed to ensure adequate uncloggingof the filter 23. As will be made clear, the program then branches toeither a step 151 or the burner deactivation step 119 (see FIG. 4).After the step 119, the program returns to the initial step 101 asillustrated in FIG. 4.

In the step 151, the CPU 54 determines whether or not the time elapsedsince the commencement of unclogging of the filter 23 exceeds a presetinterval Tref7 preferably chosen to be 3 minutes. If this elapsed timedoes not exceed a preset interval Tref7, the program returns to theinitial step 101 (see FIG. 4). If the elapsed time exceeds the presetinterval Tref7, the program advances to the burner deactivation step 119(see FIG. 4) and then returns to the initial step 101.

FIG. 8 is a detailed flowchart of the temperature control step 150. In astep 161 following one of the steps 142 and 149 (see FIG. 7), the CPU 54determines a desired basic value of the pulse-width of the currentpulses driving the fuel valve 30 on the basis of the engine speed valueN and the engine load value Q in the same way as in the step 146 (seeFIG. 7). After the step 161, the program proceeds to a step 162.

In the step 162, the current value of the temperature is derived fromthe temperature signal VT. In this flowchart, a variable VT2 representsthis temperature value. Then, the CPU 54 determines whether or not thetemperature value VT2 exceeds a preset level Href3 corresponding to aminimum temperature for reliable unclogging of the filter 23. Thisreference level Href3 is preferably 550° C. If the temperature value VT2does not exceed the preset level Href3, the program proceeds to a step172. If the temperature value VT2 exceeds the preset level Href3, theprogram proceeds to a step 163.

In the step 172, the control unit 50 activates the glow plug 29 as inthe step 143 (see FIG. 7). This activation of the glow plug 29 is tofacilitate combustion of the air-fuel mixture.

In the step 173 subsequent to the step 172, the CPU 54 determines adesired final value of the pulse-width of the current pulses driving thefuel valve 30 on the basis of the desired basic pulse-width value givenin the step 161. Specifically, the desired final pulse-width value isequal to the corresponding basic value multiplied by a factor of 1.6.After the step 173, the program advances to a step 174.

In the step 174, the CPU 54 determines whether or not the time elapsedsince the moment at which the temperature value VT2 first dropped belowthe reference level Href3 in the step 162 or since the moment at whichthe temperature value VT1 first rose above the reference level Href2 inthe step 148 (see FIG. 7) exceeds a preset interval Tref8. Thisreference interval Tref8 is preferably 15 seconds. If this elapsed timeexceeds the preset interval Tref8, the program advances to the burnerdeactivation step 119 (see FIG. 4) and then returns to the initial step101 (see FIG. 4). If the elapsed time does not exceed the presetinterval Tref8, the program advances to a step 171.

In the step 163, the control unit 50 deactivates the glow plug 29 as inthe step 149 (see FIG. 7). After the step 163, the program advances to astep 164.

In the step 164, the CPU 54 determines whether or not the temperaturevalue VT2 exceeds a preset level Href4 preferably chosen to be 580° C.If the temperature value VT2 does not exceed the preset level Href4, theprogram advances to a step 165. If the temperature value VT2 exceeds thepreset level Href4, the program advances to a step 166.

In the step 165, the CPU 54 determines a desired final value of thepulse-width of the current pulses driving the fuel valve 30 on the basisof the desired basic pulse-width value given in the step 161.Specifically, the desired final pulse-width value is equal to thecorresponding basic value multiplied by a factor of 1.4. After the step165, the program proceeds to the step 171.

In the step 166, the CPU 54 determines whether or not the temperaturevalue VT2 exceeds a preset level Href5 preferably chosen to be 600° C.If the temperature value VT2 does not exceed the preset level Href5, theprogram advances to a step 167. If the temperature value VT2 exceeds thepreset level Href5, the program advances to a step 168.

In the step 167, the CPU 54 determines a desired final value of thepulse-width of the current pulses driving the fuel valve 30 on the basisof the desired basic pulse-width value given in the step 161.Specifically, the desired final pulse-width value is equal to thecorresponding basic value multiplied by a factor of 1.2. After the step167, the program proceeds to the step 171.

In the step 168, the CPU 54 determines whether or not the temperaturevalue VT2 exceeds a preset level Href6 preferably chosen to be 620° C.If the temperature value VT2 does not exceed the preset level Href6, theprogram advances to a step 169. If the temperature value Href6 exceedsthe preset level Href6, the program advances to a step 170.

In the step 169, the CPU 54 determines a desired final value of thepulse-width of the current pulses driving the fuel valve 30 on the basisof the desired basic pulse-width value given in the step 161.Specifically, the desired final pulse-width value is equal to thecorresponding basic value multiplied by a factor of 1.0. After the step169, the program advances to the step 171.

In the step 170, the CPU 54 determines a desired final value of thepulse-width of the current pulses driving the fuel valve 30 on the basisof the desired basic pulse-width value given in the step 161.Specifically, the desired final pulse-width value is equal to thecorresponding basic value multiplied by a factor of 0.8. After the step170, the program advances to the step 171.

In the step 171, the control unit 50 energizes the fuel valve 30intermittently at the fixed frequency while maintaining the duration ofeach energization cycle of the fuel valve 30 at a value equal to thedesired final pulse-width value determined in the steps 165, 167, 169,170, or 173. Specifically, the control signal S2 in the form of a pulsetrain having a constant frequency equal to the fixed value and having apulse-width equal to the desired final pulse-width value determined inthe steps 165, 167, 169, 170, or 173 is outputted to the controlterminal of the switch 51b so that the switch 51b is closedintermittently at the same frequency and the duration of each cycle ofclosure of the switch 51b matching the desired final pulse-width value.This periodic closing of the switch 51b actuates a correspondinglyperiodic opening of the fuel valve 30. Opening the fuel valve 30 allowsfuel supply to the burner 25. It should be noted that air supply to theburner 25 is allowed by the step 146 (see FIG. 7). After the step 171,the program advances to the step 151 (see FIG. 7).

The rate of fuel supply to the burner 25 determined by the desired finalpulse-width value varies as a function of the engine speed and load.Furthermore, as a result of execution of the steps 164, 165, 166, 167,168, 169, and 170, the rate of fuel supply to the burner 25 decreaseswith increases in the temperature value VT2. Accordingly, thetemperature in the burner 25 is maintained within a range, preferably of600°-620° C., which is suitable for reliable unclogging of the filter23.

FIG. 9 is a detailed flowchart of the burner deactivation step 119 ofFIG. 4. In a step 181 following one of the steps 102 and 118 (see FIG.4), the control unit 50 de-energizes the fuel pump 33. Specifically, thecontrol signal S1 designed to open the switch 51a is outputted to thecontrol terminal of the switch 51a. Opening the switch 51a results inde-energization of the fuel pump 33.

In a step 182 subsequent to the step 181, the control unit 50 holds thefuel valve 30 closed. Specifically, the control signal S2 designed toopen the switch 51b is continuously applied to the control terminal ofthe switch 51b. Opening the switch 51b results in closure of the fuelvalve 30. Accordingly, fuel supply to the burner 25 is interrupted.

In a step 183 subsequent to the step 182, the control unit 50de-energizes the air valve 37 (see FIG. 1). Specifically, the controlsignal S3 designed to open the switch 51c is outputted to the controlterminal of the switch 51c. Opening the switch 51c results inde-energization of the air valve 37. This interrupts air supply to theburner 25. After the step 153, the program returns to the initial step101 (see FIG. 4).

What is claimed is:
 1. An exhaust particle removing system for an internal combustion engine, comprising:(a) a filter disposed in an engine exhaust passage for trapping particles suspended in exhaust gas; (b) a burner for burning off the particles deposited on the filter; (c) means for sensing the pressure in the exhaust passage at a point upstream of the filter; (d) means for sensing the pressure in the exhaust passage at a point downstream of the filter; (e) means for determining whether or not the sensed upstream pressure is lower than a preset level; (f) means for, when the sensed upstream pressure is not lower than the preset level, deducing the degree of clogging of the filter on the basis of the sensed upstream and downstream pressures; (g) means for, when the sensed upstream pressure is lower than the preset level, measuring a time elapsed since the moment at which the sensed upstream pressure dropped below the preset level; (h) means for, when the sensed upstream pressure is lower than the preset level, deducing the degree of clogging of the filter on the basis of the time elapsed and the sensed upstream and downstream pressures obtained immediately prior to the moment at which the sensed upstream pressure dropped below the preset level; and (i) means for controlling the burner on the basis of the deduced degree of clogging of the filter.
 2. The system of claim 1, further comprising means for sensing an operating condition of the engine, the deduced degree of clogging of the filter being also dependent on the sensed engine operating condition when the sensed upstream pressure is lower than the preset level.
 3. The system of claim 2, wherein the sensed engine operating condition includes rotational speed of the engine.
 4. The system of claim 2, wherein the sensed engine operating condition includes load on the engine.
 5. The system of claim 1, further comprising means for sensing rotational speed of the engine, and means for sensing load on the engine, and wherein the deduced degree of clogging of the filter is also dependent on the sensed engine speed and load when the sensed upstream pressure is lower than the preset level.
 6. In an exhaust particle removing system including a filter disposed in an engine exhaust passage for trapping particles suspended in exhaust gas, a method comprising the steps of:(a) sensing the pressure in the exhaust passage at a point upstream of the filter; (b) sensing the pressure in the exhaust passage at a point downstream of the filter; (c) determining whether or not the sensed upstream pressure is lower than a preset level; (d) when the sensed upstream pressure is not lower than the preset level, deducing the degree of clogging of the filter on the basis of the sensed upstream and downstream pressures; (e) when the sensed upstream pressure is lower than the preset level, measuring a time elapsed since the moment at which the sensed upstream pressure dropped below the preset level; (f) when the sensed upstream pressure is lower than the preset level, deducing the degree of clogging of the filter on the basis of the time elapsed and the sensed upstream and downstream pressures obtained immediately prior to the moment at which the sensed upstream pressure dropped below the preset level; and (g) removing the particles from the filter in accordance with the deduced degree of clogging of the filter. 